The present invention relates generally to a process for the production of a filter material and more particularly a filter material comprising an open-pore foam carrier with adsorber particles of different sizes fixed thereon.
One form of process for producing a filter material comprising an open-pore foam carrier with adsorber particles fixed thereto, as disclosed in German published specification (DE-AS) No 28 04 154, provides that the foam carrier material and the surface of the pores thereof are provided in a first operation with adsorber particles which are very small in comparison with the mean pore diameter of the foam carrier. Porous adsorber particles are then additionally introduced into the pores of the foam carrier. Those porous particles are relatively large, in comparison with the adsorber particles first applied to the foam carrier. The operation of introducing the porous adsorber particles, in the form of grains which substantially fill the pores in the foam carrier, is effected by the adsorber particles or grains being shot into the foam carrier by means of a directed air flow. It can be considered therefore that the relatively large porous adsorber particles are fixed in the pores primarily by a form of mechanical clamping or retaining effect. However, fixing the relatively large and porous adsorber particles in that way suffers from various difficulties. For that reason the above-quoted publication also states that it would be possible for the foam carrier with the relatively large adsorber particles embraced therein subsequently to be impregnated with a weak solvent which results in the wall surfaces of the pores in the foam being softened so that the adsorber particles or grains which are applied thereto are caused to adhere to the wall surfaces of the pores. The relatively larger grains may be also be secured by a heat fixing operation. However, irrespective of the way in which the relatively large adsorber particles or grains are fixed in the pores in the foam carrier, that process always provides that the pore surfaces are firstly at least partially covered with the first and relatively small adsorber particles, whereafter the relatively large and porous adsorber particles are introduced into the open pores. If however the first smaller adsorber particles do not completely cover the surfaces of the pores, the the adsorption qualities of the resulting filter material still leaves something to be desired. If on the other hand the first smaller adsorber particles completely cover the surfaces of the pores, then although the filter material which is produced in that way may have good adsorption properties, the second larger adsorber particles are disposed on the first and previously deposited smaller adsorber particles over at least a certain area of contact therebetween, so that that part of each of the first smaller adsorber particles is unable to take an active part in the adsorption process. Furthermore it often transpires that the larger adsorber particles are not satisfactorily fixed to the smaller adsorber particles on the foam carrier material.